The present invention relates to a liquid development apparatus which uses a liquid developer containing a toner in the liquid and an image formation apparatus using the same. More specifically, the present invention relates to a liquid development apparatus or the like which develops an electrostatic latent image formed by a method such as electrophotography, electrostatic recording or ionography.
Conventionally, in an image formation apparatus which forms an image by using an electrophotographic method, there is one which develops an electrostatic latent image formed on a latent image supporting body by a liquid developer in which a toner is dispersed in a nonconductive liquid. With this type of image formation apparatus, the charge quantity of electric charge of the toner particles in an electric field easily changes between the initial stage and a stage after the lapse of time. In order to obtain a stable image having a little change in image quality, it is necessary to suppress a change in the charge quantity of the toner.
Therefore, various contrivances have been heretofore made for controlling the charge quantity in the liquid developer and the liquid developing process.
For example, as the one which exemplifies a liquid developer having excellent toner charging stability, redispersibility and charging stability at the time of redispersion, and capable of preservation for a long period of time and reuse, there can be mentioned Japanese Patent Application Laid-Open No. 2000-181148 and Japanese Patent Application Laid-Open No. 2000-181149. In these liquid developers, the composition is limited in order to exhibit desired charging stability and the like. However, there is an instance when a liquid developer comprising a different composition needs to be used, depending on the type of the liquid development apparatus.
As a method for stabilizing charging in the nonconductive liquid of the toner, there is known a method of monitoring the charge quantity and controlling the charge quantity by adding a charge control agent depending on the condition thereof. With this method, however, the mechanism becomes large and complicated because of installing a charge control agent replenishment mechanism. Further, a change with the lapse of time including a weight ratio of the charge control agent in the liquid developer has to be taken into account, causing a problem in that the construction for controlling the charge quantity becomes more complicated.
As a result of investigation relating to the cause of the change with the lapse of time of the charge quantity of the toner, the present inventors have found the followings. That is, the change with the lapse of time of the charge quantity includes an increasing case and a decreasing case, but the toner in the nonconductive liquid changes the electric charge to decrease frequently. In the liquid developer in which the toner is dispersed in the nonconductive liquid, the electric charge decreases with the lapse of time, as shown in FIG. 1. A and B in FIG. 1, there are shown toners having a difference in the formula of a coloring material and a resin material. It is seen that there is a difference in degree of the decreased amount of the electric charge according to the formula (toners A, B), but the decreased amount in both cases decreases with the lapse of time. In FIG. 1, the lapse of time is shown only for 20 days, but the decrease in the electric charge saturates about one month later. If the toner in the liquid developer coheres, a decrease in the electric charge of the toner can be seen, but in the toners A and B in the figure, cohesion does not occur. It is because, as shown in FIG. 2, any difference cannot be recognized in the particle size distribution of the toner between the initial stage and a stage after the lapse of time. For the reason why the electric charge of the toner decreases even though cohesion does not occur in the toner, the chemical state of the liquid developer is involved. Specifically speaking, when the degree of the chemically non-equilibrium state of the liquid developer increases, a deviation occurs in the polarity to increase the electric charge of the toner. On the other hand, when the liquid developer approaches the chemical equilibrium state, the electric charge of the toner decreases. With regard to the liquid developer in which the electric charge of the toner is decreased resulting from approaching to the equilibrium state, it is theoretically possible to return the electric charge of the toner to the original state by causing the chemically non-equilibrium state by applying an appropriate stress. However, it is necessary to increase the stress considerably. Specifically, in the conventional liquid development apparatus, a stirring member is provided for stirring the liquid developer, and a stress is given to the liquid developer by stirring. However, this stirring is performed for the purpose of making the toner density uniform in the liquid developer, and the stirring force is set to be weak which is sufficient for making the toner density uniform. With this level of stirring, it is difficult to sufficiently generate the chemically non-equilibrium state. The variation character of the electric charge of the toner shown in the figure shows a typical example, and the variation character thereof is not limited to the one shown in the figure. In the graph of FIG. 2, the ordinate on the left side from the dotted line in the figure is indicated by frequency (%), and the ordinate on the right side from the dotted line in the figure is indicated by accumulated total (%).
From this research result, people tend to jump to a conclusion that if the stress imparted to the toner (liquid developer) is increased than the conventional value, a decrease in the electric charge of the toner with the lapse of time can be dissolved. However, if the stress is increased, the toner is easily crushed by this. Also there is the possibility that the electric charge of the toner may be decreased by this crush. Further, not only the value (strength) of the stress, but also the stress application time take part in the crushing of the toner. With an increase of the stress, the toner is crushed with a shorter period of stress application time. Therefore, in order to increase the stress to the degree sufficient for generating the chemically non-equilibrium state, it is necessary to adequately control the stress application time, in addition to how much degree the stress applied to the liquid developer is increased.
However, it is very difficult to adequately control the stress application time with respect to the liquid developer in the apparatus. This is due to the reasons explained below. That is, in FIG. 1, the variation character of the electric charge of the toner in the liquid developer in which the toner is not replaced is shown, but in the actual apparatus, the toner often goes in and out with respect to the liquid developer. Specifically, the liquid development apparatus transports the liquid developer supported on a developing member such as a developing roller to a position opposite to a latent image supporting body to perform development of the latent image. Then, the liquid developer remaining on the developing member which has passed through the opposed position is collected and reused. The collected liquid developer has consumed the toner and the nonconductive liquid accompanying the development, and hence the toner density therein is changed from the initial state. Therefore, if the liquid developer is returned to the liquid developer in the apparatus, the toner density thereof will be changed. Therefore, the toner density of the liquid developer in the apparatus is monitored by a sensor, and a liquid developer and/or nonconductive liquid of high density is replenished into the apparatus, depending on the result, thereby the toner density in the liquid developer within the apparatus is stabilized. As described above, in the actual apparatus, the toner goes out from the liquid developer due to the toner consumption accompanying the development, or new toner comes into the liquid developer due to the replenishment of the liquid developer of high density. Since the electric charge as the whole toner in the liquid developer changes regardless of the lapse of time due to going in and out of the toner, the electric charge of the toner cannot be grasped based on the lapse of time. Hence, it is difficult to adequately control the stress application time.
In the Japanese Patent Application Laid-Open No. 2000-181148, there is disclosed an image formation apparatus provided with an attriter as a stress application unit which applies stress loading to the liquid developer. According to the experiments performed by the present inventors, this attriter could generate the chemically non-equilibrium state with respect to the liquid developer to thereby recover the electric charge of the toner. However, in this publication, the stress application time by the attriter is not taken into consideration. Therefore, there is the possibility that the toner may be crushed by the attriter, to thereby decrease the electric charge thereof.
It is an object of the present invention to provide a liquid development apparatus and an image formation apparatus which can reliably stabilize the charge quantity in a liquid developer, while preventing that the construction of the apparatus becomes complicated due to an attached charge control agent replenishment mechanism.
The present invention is a liquid development apparatus which has a developer storage section which stores a liquid developer containing a toner in the liquid, and a stress application unit which applies stress loading to the liquid developer in the developer storage section, and develops a latent image formed on a latent image supporting body in an image formation apparatus by the liquid developer, wherein a charge quantity detection unit which detects the toner charge quantity of the liquid developer in the developer storage section is provided.
The present invention is an image formation apparatus comprising a liquid development apparatus having a developer storage section which stores a liquid developer containing a toner in the liquid, and a stress application unit which applies stress loading to the liquid developer in the developer storage section, and a control unit which controls at least this liquid development apparatus, and develops a latent image on a latent image supporting body by the liquid development apparatus, wherein the control unit is constructed so as to control the operation of the stress application unit, based on the charge quantity change information showing the change of the toner charge quantity in the liquid developer in the developer storage section.
Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.